Due to the shrinkage of microcircuit dimensions, current and future semiconductor fabrication technologies, such as film deposition, lithography, film etch, and chemical mechanical planarization, require ever increasing process quality control and process yield. Moreover, the increase in wafer size from 200 mm to 300 mm also increases the demand for tighter process control because more chips are fabricated in a single wafer and, accordingly, losing a single wafer due to a process failure results in a higher loss.
To achieve better process control and improve fabrication yields, semiconductor manufacturers are adopting metrology systems as an integral part of their process control. The metrology system can measure and provide a wafer parameter, such as the thickness of a deposited thin film, to the process tool, which can adjust a process parameter to either correct the processed wafer in a subsequent process step or correct the previous process step for subsequent wafers. The technology of measuring a given parameter (or many parameters) and feeding back and/or forward the information to the process is generally known as Advanced Process Control (APC).
For faster information feed back and therefore better yield, the industry is adopting what is known as Integrated Metrology®. Integrated Metrology® (IM) systems are small-footprint, high precision measurement tools, which can be physically attached to the process tool and are an integrated part of the manufacturing process. In relation to Stand Alone (SA) metrology systems, IM systems have several advantages when used for APC. For example, through the use of IM, the time between the end of a process and measurement for verification is drastically reduced, which reduces the APC cycle time by permitting the measured parameters to be fed back or forward into the process cycle much earlier than with a SA system. A reduced APC cycle time results in tighter process control from wafer to wafer, or even from lot to lot. Further, because a deviation from the specified target parameter is detected earlier with an IM system, wafers that could potentially be miss-processed are saved due to the faster APC cycle time. Finally, an added advantage is the lower exposure of wafers to risks such as breakage and contamination, since the integration of the metrology unit into the process tool reduces wafer traffic compared to the wafer transportation necessary with SA systems.
The demand for system reliability in an IM system, however, is higher than for an SA system, because the IM system forms an integral part of the process cycle or production line. In fact, an IM system requires increased system reliability in terms of both the number of failures and measurement performance degradation. The failure of the IM system may stop the production line for many hours until service is performed on the tool. Measurement performance degradation of the IM system may result in incorrect information being fed back or forward to the process tool. The process tool will then unnecessarily adjust process parameters based on the incorrect information, which could result in many wafers being miss-processed.
Accordingly, system checks are routinely performed on a daily or monthly basis to assure measurement reliability of the IM systems. Common system checks include precision, stability and accuracy. For the case of thin film and critical dimension (CD) metrology tools based on optical spectroscopy, such as reflectometry or ellipsometry, it is common to use wafers that have standardized films or CD features, made of stable materials such as Silicon Oxide (SiO2) to perform the system checks. These standards are pre-characterized using a high accuracy system. System checks for IM systems require the introduction of the standard wafer into the process tool, which is disadvantageous because it interferes with the process cycle and reduces the number of slots available to production wafers.
Other types of optical metrology tools, such as those that measure overlay error, reflectivity, color, defects, and non-optical metrology tools, such as those that measure film capacitance, conductive film thickness, conductive film sheet resistance, conductive film inductance, conductive film magnetic properties, geometric shape, and vertical step height, may similarly benefit from the use of standard wafers with pre-characterized features in system checks.
Accordingly, there is a need for improved system checks for metrology systems and, in particular, for metrology systems that will minimize the interference with the process cycle.